Page 93 - The Jet Engine

Lubrication

invariably of tubular construction with a pleated being able to survive in an engine environment which
woven wire cloth, or a resin impregnated with fibres, exhibits very high temperatures. Having met these
as the filtering medium. Some filters comprise one or fundamental requirements, the need to provide
more wire wound elements but these tend to be improved lubrication characteristics using additives
insufficient for fine filtration. A 'pop up indicator' may must also be investigated. Special laboratory and
be fitted to the filter housing to give a visual warning engine tests are done to prove the suitability of a
of a partially blocked filter. particular oil for a specific type of engine.
Assessments are made as the extent to which it
LUBRICATING OILS deteriorates and the corrosive effects it may have on
the engine.
31. Early gas turbines used thinner oils than those
used in piston engines but were produced from the 33. Most gas turbines use a low viscosity oil due to
same mineral crude oil. As gas turbines were the absence of reciprocating parts and heavy duty
developed to operate at higher speeds and tempera- gearing. This reduces the power required for starting,
tures these mineral oils oxidized and blocked the particularly at low temperatures. In fact normal starts
filters and oilways. The development of low viscosity can be made in temperatures as low as -40 deg. C.
(thin) synthetic oils overcame the major problems without having to pre-heat the oil.
encountered with the early mineral oils.
34. Turbo-propeller engines use a slightly higher
32. The choice of a lubricating oil is initially decided viscosity oil due to the additional requirements of the
by the need to start the engine at very low tempera- reduction gear and propeller pitch change
tures, when the viscosity of the oil is high, whilst mechanism.

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